Just get a higher basking spot item. You'll likely need a lower wattage heat lamp as well once your raise the basking platform. You can also look at ways to mount the UVB inside the tank. Your tank is so tall though, I would get higher items to provide more exercise and fun for the little bugger.
I encourage everyone to get their dragons tested for ADV (Adenovirus). You can get that done through here:
http://www.thepats.info/sign_up.html
Tracey runs this project. Knowing the ADV status of your dragon will help you understand his needs in the future. Petco/PetSmart sources their reptiles from mills. They are severely abused there and our dragons would come in (I used to work at a Petco and a PetSmart) frequently with Yellow Fungus and Adenovirus. ADV is very common in dragons nowadays and it's important to their status as it will determine whether or not they can be around other dragons, potential life-span, and care needed down the line. Dragons that have it but live through adolescence will often suffer from liver and intestinal damage and the virus can spread to the brain and spinal column. So just know their status and take care of them accordingly.
I got my dragon from a well known and respected breeder and he even has ADV, so I give him supplemental slurries. I just like to throw it out there as increasing the understanding of ADV and making it a more prevalent issue is something very important to me. And my dragon would not be where he is today, health-wise, if I hadn't known he had ADV.